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> 1,000 Words: Roof Top Units (RTUs) and Economizers

Posted by Lester Shen  |  Date August 8, 2013  |  Comments 0

On most commercial buildings you will find roof top units (RTUs), air handlers that provide heating, cooling, and ventilation to the building’s conditioned spaces. Heating and air conditioning account for over 25 percent of the energy use of commercial buildings, and rooftop units serve 40 percent of commercial floorspace.

RTU.JPGBecause RTUs represent such a large piece of commercial energy consumption, CEE works to enhance their efficiency through our projects and programs. In order to disseminate research findings and recruit program participants, we have found that it’s essential to clearly explain how these systems work. Under the guidance of Senior Mechanical Engineer Mark Hancock, we’ve produced the following animations to show how RTUs and economizers work. 

Cooling and Heating

When the space calls for cooling, a blower in the RTU draws warm air from the space below, through the cooling coil, and then supplies the space below with the cooled air.

A similar process occurs to provide heated air.  



RTUs also ventilate the space, always supplying some outside air. Notice in the animation below that the outside air damper is no longer fully closed, which allows the RTU fan to bring in a minimum amount of outside air. Notice the addition of pressure relief for the added outside air.

A space with people in it requires more ventilation than when it’s unoccupied. An occupancy sensor or a CO2 sensor can respond to changes in ventilation demand, controlling the outside air damper and bringing in more air as necessary.


RTU and Economizer Operation

An economizer is a control system that takes advantage of outside air conditions to save energy when cooling is called for in the conditioned space. When the outside air temperature is lower than the space temperature, the economizer opens the outside air damper of the RTU and brings in cool air. This saves energy, because otherwise the RTU would cool the inside air mechanically.

The following animation shows the standard operation of an RTU with an economizer. It also shows the operation of the compressor that is connected to the cooling coil. In standard operation, both the compressor and the fan run at a constant speed.

The final animation shows optimized operation of the RTU, where the fan and compressor speeds are modulated based on the conditioning demand of the space. Under normal conditions, this approach leads to the most energy efficient operation of the RTU.

Feel free to link to or embed these videos from this blog post, our resource page, or our YouTube channel.

The motion graphics animations were designed and produced by Huma Saqib.

*This research is supported in part by a grant from the Minnesota Department of Commerce, Division of Energy Resources through the Conservation Applied Research and Development (CARD) program. And with co-funding by CEE in support of its nonprofit mission to advance research, knowledge dissemination, and program design in the field of energy efficiency.

Related posts:

Roof Top Unit Optimization Pilot Project: A Wrap Up

Building Automation System Overview

> 1,000 Words: Combi System Animations


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